Thermally curable adhesives typically present a trade-off between cure temperature and/or cure speed versus shelf-life. Generally, the more chemically active a thermally curable adhesive, the shorter its shelf-life at ambient temperatures.
Numerous attempts have been made to try to overcome this problem and achieve a balance of these two characteristics, although none have yet proved to be satisfactory. One approach to improving the shelf-life stability of a thermally curable adhesive is triggered reactivity. In this approach, a chemical or physical change occurs to the adhesive upon the application of some activation step, or “trigger.” Examples of such triggers for one-part systems include exposure to moisture (e.g. moisture cure one-part polyurethanes or silicones), thermal deblocking of a reactive species or curing agent (e.g. blocked isocyanate curing agents for polyols or salted/adducted amine curing agents for epoxies), exposure to radio frequency radiation, internal heating of an adhesive containing ferromagnetic particles upon exposure to a fluctuating magnetic field (induction cure), or ultra-violet (UV) or visible radiation to photochemically initiate a reaction (radiation curing).
One feature of radiation curable materials is that the cure is localized to the areas exposed to radiation (i.e. the UV or other light source). For applications in which two opaque substrates are to be bonded together, a traditional radiation cure is often not an option as light cannot reach the adhesive bond line through those opaque substrates.
Cationic UV curing has been employed in some cases for radiation curing between two opaque substrates. Cationic UV systems typically function via photochemically induced formation of a strong acid from a cationic initiator such as a diaryliodonium or a triaryl sulfonium salt.
With common cationic UV curable systems, these photoinitiators are combined with highly reactive epoxies, vinyl ethers, or oxetanes. The strong acid produced by the photoinitiator upon light exposure induces a rapid cationic cure of these acid-reactive monomers and oligomers to form a polymerized/crosslinked resin matrix. Typical liquid cycloaliphatic epoxies, vinyl ethers, and oxetanes cure with kinetics similar to radically curable systems, in that there is little or no open time or latency between UV irradiation and extensive resin cure. However, components or additives which essentially reduce the activity of the photoacid catalyst or the cationic propagating species can slow or retard cationic cure because the propagating cation is not quenched by typical ambient conditions that would terminate the active species in a radical polymerization. Several approaches are known to retard the cure of UV curable cationic systems, but all have attendant drawbacks.
In some cases, a photoinitiator is used with a nucleophilic anion. However, while increasing the nucleophilicity of the counteranion slows down the kinetics of the cure generally, it does not provide a noticeable change in latency or discrete open time between activation and cure. As such, this approach has limited commercial utility.
Another approach to delayed cationic cure is the use of mildly basic additives, such as aliphatic or aromatic amines. However, this approach is also unsatisfactory because the basic additive remains present in the final cured network, resulting in possible migration or extraction during end use, as well as increased sensitivity to ambient impurities and reduced cure rates upon heating.
Still another way to delay or retard a cationic cure is to use a less reactive polymerizable resin. The simple use of low reactivity liquid monomers is of limited utility unless only very brief open times are required. There is not a true latency period in which propagation is slowed or stopped, but some open time can be attained due to the overall slow cure kinetics of the liquid adhesive. In this situation there is not a cure “trigger” which suddenly accelerates the system reactivity/cure kinetics. The inherent liquid adhesive reactivity is simply low.
An extension of systems with slow cationic cure kinetics is so-called “frontal polymerization,” in which the activation energy for ring-opening polymerization exceeds ambient thermal energy and the curing process does not proceed until a point source of heat is applied, at which point thermal autoacceleration occurs due to the exotherm of polymerization. However, unless heat is actively removed from the liquid system after autoacceleration begins, it cannot be slowed or stopped, so there is not an open time between triggering/activating the system and the onset of rapid curing.
Furthermore, frontal polymerization and slow monomer delayed UV cationic cure concepts rely completely on specific structural motifs present in the particular epoxy monomers used. As such, these techniques are useful for a limited subset of monomer structures, all of which are liquids.
Two part initiator systems have been combined with slow cure kinetics and frontal polymerization concepts, but such systems still require mixing at the point of use. That mixing often presents significant practical manufacturing limitations, with latency on the order of a few seconds to an hour.
These and other drawbacks are associated with conventional thermally curable adhesives.